1. Technical Field
The present invention relates to a separator for a lithium second battery and a lithium secondary battery comprising the same, and more particularly, to a separator for a lithium secondary battery that has a shutdown function and excellent high-temperature shape stability, and a lithium secondary battery comprising the same.
2. Description of the Related Art
With the rapid development of electronic, communication and computer industries, mobile electronic communication equipments, such as, for example, camcorders, mobile phones, laptop computers, and the like have been advancing remarkably. Accordingly, the demand for lithium secondary batteries as a power source of mobile electronic communication equipments is increasing day by day. In particular, recently, research and development has been actively made all over the world including Japan, Europe, USA as well as Korea to develop lithium secondary batteries as an environment-friendly power source of large-scale equipments, such as, for example, electric vehicles, uninterruptible power supplies, electromotive tools, satellites, and the like, as well as lithium secondary batteries as a power source of mobile electronic equipments.
Generally, a lithium secondary battery includes a cathode of lithium-transition metal composite oxide, an anode capable of intercalating or disintercalating lithium ions, a separator interposed between the cathode and the anode, and an electrolyte that helps the migration of lithium ions.
The main function of the separator is to separate the cathode from the anode and hold the electrolyte therein to provide high ion permeability. Recently, a separator having a shut-down function has been suggested, in which, for example, when a short circuit occurs in a battery, the separator melts to stop the pores so as to keep a large amount of electric current from flowing into the battery. Also, suggestions have been made to increase the area of a separator larger than those of cathode and anode plates to prevent the cathode and anode plates from contacting. However, in this case, the two electrode plates may contact when the separator shrinks with the increasing internal temperature. An additional function is required to prevent an internal short circuit caused thereby. In particular, in the case of a modern high capacity and high energy density lithium secondary battery, a separator having higher heat resistance and thermal stability than a conventional separator is needed because the temperature in the battery increases to a higher level than that of a general battery when the battery is repeatedly charged and discharged at a high rate.
Conventionally, a porous membrane formed of a sheet using a polyolefin-based polymer, such as, for example, polyethylene (PE) or polypropylene (PP) has been widely used as a separator. A separator made of polyethylene having a melting temperature of 130° C. or polypropylene having a melting temperature of 170° C. thermally shrinks/melts due to the heat generated when an excessive amount of electric current flows into a battery by a short circuit or an increase in the internal temperature by the effect of a certain external factor, and consequently stops the micropores to shut down the flow of electric current.
In addition to a shut-down function, shape stability is another important quality of a separator. When the temperature continues to increase above the melting temperature of polyethylene or polypropylene by internal/external factors even subsequent to shut-down, the separator melts and loses shape, which causes a short circuit of electrodes.
To solve this problem, a composite separator for a lithium secondary battery has been developed. For example, Japanese Patent Publication No. 2003-22794 discloses a jelly roll-type battery comprising a separator having a melt-bonded portion to suppress the shrinkage of the separator, thereby preventing a cathode and an anode from contacting. Also, Korean Patent Publication No. 2005-0066652 teaches a lithium secondary battery with different types of separators made of materials having different temperature characteristics, in which first and second separators surrounding an electrode assembly suppress a chain reaction at the anticipated time of an internal short circuit caused by the shrinkage of the separator as a consequence of the increasing temperature in the battery under high temperature condition, thereby ensuring the safety of the battery. However, this art needs additional heat fusion and repeated overlapping of separators during cell assembly, which results in a reduction in the procedural efficiency.
Korean Patent Publication No. 2002-0001035 discloses a composite membrane that is prepared by coating a non-porous film with an active material. However, a support layer has a relatively low melting temperature of 166.5° C. and the composite membrane has a relatively high air permeability of 560 sec/100 cc, which causes deterioration in the charging/discharging characteristics.
US Patent Publication No. 2006/0019154 A1 suggests a heat-resistant polyolefin-based separator with a porous heat-resistant resin thin film, in which a polyolefin-base separator is soaked in a solution of polyamide, polyimide, or polyamideimide having a melting point of 180° C. or more and then immersed in a coagulation solution to extract a solvent, thereby achieving small heat shrinkage, good heat resistance, and good cycling performance.
Japanese Patent Publication No. 2005-209570 discloses a polyolefin separator with a heat-resistant resin layer in which both surfaces of the polyolefin separator are coated with a solution of heat-resistant resin having a melting point of 200° C. or more, such as, for example, aromatic polyamide, polyimide, polyether sulfone, polyether ketone, or polyetherimide, followed by immersion in a coagulation solution, washing, and drying, so as to ensure sufficient safety under high energy density and high capacity conditions. However, the pores of the polyolefin separator get closed due to the impregnation with the heat-resistant resin, and thus the migration of lithium ions therethrough is limited, resulting in deterioration in the charging/discharging characteristics.
WO 2004/031466 relates to an improved laminate of aramid papers and a polyester polymer layer, preferably a laminate of two aramid papers separated by a polyester polymer layer. Also, WO 2006/101243 discloses a composite sheet with a shut-down function and high-temperature shape stability suitable as a separator for secondary batteries and capacitors, that has a layered structure of a porous thermoplastic polymer sheet layer having a melting point of 200° C. or below and at least two non-woven fabric sheet layers of an organic compound not substantially having a safety melting point stacked on top of each other. However, the composite sheet of this art has a procedural difficulty in manufacturing and may have an uneven contact surface, which may increase the contact resistance of electrodes.
WO 2004/030909 discloses a method for forming a laminate of at least two layers of aramid paper and at least one layer of polymer, including calendering the opposing surfaces of the aramid paper at different temperatures before forming a laminate. However, because the aramid paper does not have a shut-down function, this art cannot shut down a flow of electric current caused by an external short circuit or heat generation, which may cause an even larger ignition.